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The present invention relates to apparatus for transmitting and receiving
ultrasound, particularly, but not exclusively, to Doppler velocimeters for
measuring the velocity of blood in blood vessels by making measurements of
Doppler frequency change in ultrasound transmitted to and received from a
blood vessel liquid whose velocity is to be measured.
In the investigation of occlusive arterial disease, assessment of the
degree of circulatory impairment can be made with the aid of an ultrasonic
Doppler velocimeter. Such velocimeters are described in: NASA Progress
Report NRG 33010074, Cornell University, 1969, "Directional Doppler
Flowmeter" McLeod, R. D.; Ultrasound in Medicine and Biology, Volume 2, 1,
"Accuracy and Limitations of the Ultrasonic Doppler Blood Velocimeter and
Zero Crossing Detector" by Lunt, M. J. (1975); and in the book
"Non-Invasive Physiological Measurements" edited by Rolfe, Academic Press,
1979, Chapter 7, "Ultrasonic Doppler Velocimetry" by Roberts and Sainz.
Briefly, in an ultrasound Doppler velocimeter an oscillator is coupled to a
transmit crystal in an ultrasonic probe, the probe being positioned so
that ultrasound is transmitted as nearly as possible along the axis of a
blood vessel in which blood velocity is to be measured. Sound reflected
from blood flowing in the vessel is received by a receive crystal in the
probe. Due to the Doppler effect a signal having a spectrum of frequencies
related to the different velocities of blood flowing in the vessel at
different times and in different parts of the cross section of the vessel
is generated. Since the frequency of insonation is usually between 2 and
16 mHZ the receive signal is demodulated using the transmit signal and
this provides a band of audio signals corresponding to the different
velocities. These audio signals can then be converted to voltages
proportional to velocity to give a convenient output signal.
In using such apparatus to make measurements of flow of blood or any other
fluid, the probe is hand held and typically houses the transmitting and
receiving crystals in a coplanar relationship, as described in the
publications already mentioned. In order to obtain the signal with the
best characteristics at any site of measurement, there is a need to adjust
the frequency at which the crystals oscillate. Such optimization frequency
can be predicted on theoretical grounds which are described in the
publications already mentioned. Typically, apparatus is used at discrete
frequencies at either 2, 5 or 10 mHz. Each separate apparatus is equipped
with a probe for each frequency and each probe contains a pair of crystals
whose frequencies are matched to the desired frequency.
According to the present inventon there is provided apparatus for
transmitting and receiving ultrasound, comprising
an electrical signal source for providing output signals at predetermined
selectable frequencies,
a frequency controller for selecting the frequency provided by the source,
and
probe means coupled to the source for transmitting ultrasound at any said
selectable frequency, and for receiving ultrasound at a frequency
relatively close to the transmitted frequency.
The main advantage of the present invention is that a single probe may be
used for all frequencies required for insonation. This is particularly
important in blood flow velocimetry since probe selection does not have to
be carried out by the operator.
The predetermined selectable frequencies may either be discrete frequencies
such as, for example, 2, 4, 8 and 16 mHz, or continuously variable
frequencies over the frequency range.
The probe means may include a plurality of pairs of piezo-electric
crystals, one transmitting and one receive crystal in each pair adapted to
operate at or relatively close to a predetermined frequency. One
surprising advantageous feature of the invention is that the transmit
crystals may be connected in parallel with one another and to the signal
source; and, similarly, the receive crystals may be connected in parallel
with one another and to means for processing signals received to provide
an output signal in a required form. Alternatively a single pair of
crystals, one for transmission and one for reception, may be provided in
which each crystal is adapted to resonate at the predetermined selectable
frequencies.
The frequency controller may include means for deriving a signal
representative of the power of the signal currently recieved. In an
indication of the received power is given, the selectable frequency which
provides most power can be determined and manually selected.
Alternatively the frequency controller may include means for selecting each
selectable frequency in turn, means for comparing the powers received when
the various frequencies are selected and means, coupled to the comparison
means, for automatically selecting that frequency which results in most
power being received.
Automatic frequency selection is clearly an advantageous feature since it
means that an operator need not select the required probe or even the
required frequency, thus allowing a relatively unskilled operator to use
the apparatus.
The processing means may include a single wide band amplifier coupled to
the receive crystal or crystals of the probe.
The processing means may also include means for demodulating the output of
the amplifier to provide a signal whose frequency and phase is indicative
of the velocity and direction of blood flow in a vessel insonated by the
probe means. Additionally the processing means may include a
frequency-to-voltage converter coupled at the output of the demodulating
means, and a scaler controlled by the frequency controller according to
the frequency transmitted by the probe to provide an output signal which
is substantially independent of the frequency of transmission of
ultrasound.
Certain embodiments of the invention will now be described by way of
example, with reference to the accompanying drawings, in which:
FIG. 1 is a block schematic diagram of an ultrasonic Doppler velocimeter
incorporating the present invention,
FIG. 2 is a block diagram of the frequency controller of FIG. 1, and
FIG. 3 is a block schematic diagram of an alternative embodiment of the
probe of the present invention.
In FIG. 1 a probe 10 contains four ultrasonic transmitting piezo-electric
crystals 11 to 14 and four ultrasonic receiving piezo-electric crystals
11' to 14'. The crystals form transmit and receive pairs in which for
example the crystals operate at, or in the case of the receive crystals
relatively close to (that is allowing for Doppler shift), the following
frequencies:
Crystals 11 and 11' - 2 mHz
Crystals 12 and 12' - 4 mHz
Crystals 13 and 13' - 8 mHz
Crystals 14 and 14' - 16 mHz
As has been mentioned above, different frequencies of insonation are
optimum under different conditions such as the position in the body and
the cross-sectional area of a blood vessel 9. The apparatus of the drawing
automatically selects a frequency at which a particular crystal pair is
resonant as described below. The number of crystal pairs is not limited
tofour but in practice it is thought that the number of such pairs for
practical uses may vary between two and six.
The transmit crystals 11, 12, 13 and 14 receive excitation signals from a
variable frequency oscillator 15 so that if one particular frequency is to
be selected the variable oscillator passes a signal at this frequency to
the probe and the crystal which is resonant at this frequency transmits
ultrasound. The oscillator 15 may itself be crystal controlled and in the
well known form in which a trimmer capacitor is connected in parallel with
a crystal. The basis of this oscillator was described in U.K. Pat. No.
537167 and it is further discussed in the paper entitled "Technical
Topics" by Pat Hawker G3VA in Radio Communication for December 1973 at
page 853. Since, as is described below, the frequency generated by the
oscillator 15 is voltage controlled, the above mentioned trimmer capacitor
is replaced by a capacitor whose capacitance varies in accordance with an
applied control voltage (such capacitors are known as Varicaps). In the
present embodiment four different voltage sources are available for
application to the Varicap, one source corresponding to each transmit
crystal.
Ultrasound signals reflected from blood flowing in insonated vessel 9 are
received by the four receive crystals 11' to 14' which are connected in
parallel to the input of a wide band amplifier 16 which covers the band of
the four frequencies available from the oscillator 15. Alternatively a
tuned amplifier may be used which has tuning components automatically
selected when the frequency of the oscillator 15 is selected.
Signals at the output of the amplifier 16 are demodulated by means of
multipliers 17 and 18 which receive reference signals from the oscillator
15 at the frequency currently applied to the crystals 11 to 14 in the
probe. The reference signal for the multiplier 18 is phase retarded by
90.degree. in a circuit 19. The signals from the multipliers 18 and 19 are
applied to amplifiers 20 and 21 whose outputs indicate by the audio
frequency produced, the velocity of blood in the vessel insonated; and by
relative phase, the direction of blood flow. The object of using the two
multipliers 17 and 18 to demodulate the signal from the probe is to give
an output which is dependent for its sign on the direction of blood flow.
A frequency to voltage converter 23 which includes a phase sensor provides
an output signal whose voltage magnitude and sign are determined by blood
flow velocity and direction, respectively. After the optimum frequency of
transmission has been selected, it is still found that the amplitude of
the received signal is proportional to the frequency of insonation and for
this reason the output signal of the frequency to voltage converter 23 is
applied to a scaler 24 which may be in the form of a variable gain
amplifier, with gain controlled according to the frequency of insonation.
The gain of this amplifier is inversely proportional to the insonation
frequency.
In order to provide the control functions mentioned below, the audio signal
to the output of the amplifier 20 is applied to a frequency control
circuit 25 which has the function of selecting the correct output
frequency for the oscillator 15 and selecting the correct gain for the
scaler 24. The controller 25 comprises a sweep circuit for sweeping the
oscillator 15 through the available transmit frequencies, a circuit for
measuring the audio power received at the output of the amplifier 20, and
a logic circuit for selecting the frequency at which the highest power is
received.
Frequency sweeping can be carried out on operation of a manual control, or
automatically either after equal time intervals or in synchronism with one
or a group of cardiac cycles. A trigger signal is developed which stems
either from the operation of the manual control or from time elapsed or
from the detection of some point in the cardiac cycle or in a group of
such cycles. This trigger signal causes a counter to step through its
various states and in each state one of the four possible voltages is
applied to the Varicap in the oscillator 15 so that one of the four
possible frequencies is generated.
A circuit which may be used as the controller 25 is shown in FIG. 2 where
the output from the amplifier 20 is applied to a moving arm of a switch 30
by way of an integrator 70. The switch 30 has five contacts 31 to 35 any
one of which can be selected by the arm. The contacts 31 to 34 are
connected to sample and hold circuits 36 to 39, respectively, which
receive a sample signal by way of contacts 41 to 44 of a switch 40.
Four reference voltages capable of selecting the four frequencies 2, 4, 8
and 16 mHz when passed to oscillator 15 are applied to terminals 56 to 59
and thence to terminals 51 to 54, respectively, of a switch 50, and
similarly four further reference voltages are applied to terminals 61 to
64 of the switch 60 by way of terminals 66 to 69, respectively.
The switches 30 to 60 are ganged together and when in manual form, if the
optimum insonation frequency is to be selected, the arms of the switches
are rotated clockwise from the contact having the lowest designation to
the contact having the highest designation. Thus in the first switch
position the reference voltage for the 2 mHz signal leaves the switch 50
causing the oscillator 15 to generate a 2 mHz signal for application to
the probe. As a result an output signal is received by the integrator 70
and after integration is applied to the sample and hold circuit 36. As the
arms of the switches are rotated through the first four positions the
output signals of the sample and hold circuits 36 to 39 represent the
powers of ultrasound received from the probe at the various frequencies.
By means of a group of interconnected level comparators shown within the
chain dotted line 71 an output signal is produced on that one of four
output lines 72 to 75 which corresponds to the frequency at which the
highest level of received ultrasound occurs. The lines 72 to 75 are used
to control gates 76 to 79 so that the reference voltage which selects the
frequency which produces the most ultrasound is applied to the terminal 55
of the switch 50. Thus when the arm of the switch 50 reaches the contact
55 the reference selected voltage is applied to the oscillator 15.
By a similar arrangement of gates 81 to 84 the selected reference voltage
appears at the output of the switch 60 for application to the scaler 24.
Within the scaler a circuit reponsive to the reference voltages selects
the required gain to ensure that all outputs from the scaler for similar
velocities are equal.
As described, an operator can select the optimum frequency at any time by
rotating the ganged switches 30 to 60 but if the switches are implemented
digitally then it is arranged that the dwell position 35, 45, 55 and 65 is
held for a time which very much exceeds the times held by the other switch
positions. When implemented digitally sweep by the switches may, for
example, be initiated by a pulse obtained from an electrocardiogram and
the sweep through the first four positions may be executed in the first
few milliseconds of the cardiac beat.
In practice of course the switches 30 to 60 are usually in solid state
form.
In the frequency-to-voltage converter 23, audio signals from the amplifier
20 are applied to a frequency shifting circuit which employs a carrier
frequency of, for example, 80 kHz. The object of carrying out this
frequency shift is to allow a relatively short time constant to be used
for smoothing. Full wave rectification at audio would require a very long
time constant much longer than the time required for a suitable sweep
through the frequencies which should be of the order of one millisecond. A
suitable frequency shifter is described under the title "The Generation of
SSB (Single Side Band) Signals" in Chapter 7--Modulation, Section 7-10 of
the book "Analysis, Transmission and Filtering of Signals", by Javid and
Brenner, published in 1973 by McGraw Hill. Briefly the frequency shifting
circuit comprises two multipliers which receive the audio signal, and the
audio signal phase retarded by 90.degree., respectively. The two
multipliers each receive an 80 kHz reference signal and the multiplier
which receives the undelayed audio signal receives the 80 kHz signal phase
retarded by 90.degree.. The outputs of the two multipliers are applied to
the inputs of a differential amplifier and the frequency shifted output is
obtained at the output of the amplifier.
A suitable frequency-to-voltage current is the phase-locked loop circuit
described in "Blood flow velocity and acceleration measurement by Doppler
shift ultrasound", Sainz, A. J., Roberts, V. C., Pinardi, G., Lindenauer,
S. M., in Non-Invasive Clinical Measurement, edit. Taylor D., Whamond, J.,
Pitman Medical, Tunbridge Wells.
The signal from the frequency shifting circuit is full wave rectified and
smoothed, using a time constant which provides smoothing over, for
example, about 5 cycles of the 80 kHz signal.
A logic circuit in the frequency controller 25 samples the smoothed
rectified output at four different times during the frequency sweep
corresponding to the reception of signals at the four different
frequencies. The sampled signals are applied to sample and hold circuits
whose outputs are coupled to comparators to allow the highest sampled
output to be identified. The logic circuit then causes the oscillator 15
to switch back to that voltage which when applied to the Varicap selects
the output frequency which produced the highest output power at output of
amplifier 20. At the same time a corresponding signal is applied to
control the gain of scaler 24.
In another embodiment of the invention illustrated in FIG. 3, the crystals
11 to 14 may be replaced by a single wide band crystal 90 in probe 10',
which for example is adapted to oscillate at four frequencies. Similarly,
the crystals 11' to 14' are replaced by a single crystal 90' formed in the
same way. Techniques of crystal cutting and choice of lattice size, which
allow a single crystal to be resonant at a number of different
frequencies, are known to crystal manufacturers.
In a development of this modification the single crystal pair which
replaces the four pairs of crystals shown in the drawing is resonant at a
large number of discrete frequencies so that it can be regarded as being
variable over the whole frequency band from, for example, 2 mHz to 16 mHz.
Additionally a Varicap trimmer may be connected across the crystal to
allow frequencies between the discrete frequencies to be used. When such a
crystal is used the voltage applied to the Varicap in the oscillator 15 is
also continuously variable and in selecting the optimum frequency a peak
in the output from the frequency shifter is detected by the logic circuit
and the time of occurrence of this peak in relation to the frequency sweep
is determined to select the optimum frequency.
It will be clear from the embodiment of the invention specifically
described that the invention can be put into practice in many other ways.
For example other forms of demodulation of the signal from the probe may
be used and in particular an "intermediate frequency" may be employed. The
frequency controller and the variable frequency oscillator may also be
implemented in other ways.
The invention can be applied in many fields, particularly where the
velocity of a fluid is to be measured, or a signal which depends on fluid
velocity is to be derived.
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